Many designs for progressive (multifocal) ophthalmic lenses are based mainly on the geometrical shape of one or more of the lens surfaces. The geometrical properties, however, are only indirectly related to the lens"" actual optical performance. For example, the curvature of any of the lens surfaces is only approximately related to the lens optical power.
The actual performance of an ophthalmic lens depends not just on the lens itself, but on the full eye-plus-lens system. This becomes particularly important, for example, when the lens user suffers from astigmatism and/or presbyopia. Astigmatism is a condition in which the eye focuses differently in different directions. Presbyopia is a condition in which the eye loses some of its ability to accommodate, i.e. to focus sharply at nearby objects. The curvature of the lens of the eye changes as the eye focuses on objects at different distances from the eye. As people age, their eyes become less elastic and therefore can change the curvature of the lens only to a certain degree.
The article by J. Loos, Ph. Slusallek and H. -P. Seidel, entitled xe2x80x9cUsing Wavefront Tracing for the Visualization and Optimization of Progressive Lensesxe2x80x9d, Computer Graphics Forum, vol. 17, no. 3, 1998, pp. 255-264, U.S. Pat. No. 6,302,540 to Katzman et al. and U.S. Pat. No. 6,382,789 to Baudart et al. describe methods for designing a lens that are based on optimizing certain classical optical characteristics such as power and astigmatism. PCT publication WO 01/92948 by Rubinstein et al. further provides a method for optimizing a lens where the optimization parameters include a control on the variations of the optical properties across the lens and a control on the astigmatism direction distribution.